If you are stuck...not so much that you have nothing to write about, but more about where your play should go from here, or why it started out really good and now you've gotten a bit lost in the dialogue, or the plot or the characters.
Take one of your characters and compose a letter that would come from them, telling you what's wrong with your play. You might be surprised on what things come out.
A suggestion is to type fast, don't think too much.
You could post it here if you like, for curiosity, not critique, or you could just let others know if this worked for you.
You never know...you might end up with an intersting monologue.
This isn't exactly along the same lines but I found it to be a useful exercise in developing caharcters. Write a letter from the point of view of your protagnist asking another character for something they need and explaining why. Then write a response refusing the request with detailed explanation of why it can't be fufilled. I know this is an old exercise but it has often helped me to clarify why a character needs sometimes and sometimes weven why the other characters refuse to help.
I'm not sure about the "typing fast" concept. I agree that it is a form of stimulating a stream of consciousness and therefore idea generation, but I think that understanding structure and what is supposed to happen next is the key. I know alot of playwrights use three act structure, which can be extended to four act structure, which can be extended to hero's journey ( http://www.clickok.co.uk/index4.html ); i think plotting your story out in this (whichever structure you prefer) way and then using each stage to generate ideas is a better method.
While I agree the three/four act and hero's journey form are vital to theatre I feel that often time the aristotelian form these are based off off can often lead to predictability. I don't think either form that or stream of concious is "better" it just depends on writing style. I for instance write mostly non-realistic one acts (often dealing with social issue topics) I find that a three act form doesn't properly reflect the protagonists journey in all cases.